Implement Dynamic Systems

This adds support for creating `DynamicSystem`s and for constructing
`GenericQuery`s that can be used to implement systems that are loaded at
runtime instead of compile time.

Cargo.toml

@@ -89,6 +89,7 @@ serde = { version = "1", features = ["derive"] }
 log = "0.4"
 ron = "0.6"
 anyhow = "1.0"
+bytemuck = "1.4.1"
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 console_error_panic_hook = "0.1.6"
@@ -206,6 +207,10 @@ path = "examples/ecs/parallel_query.rs"
 name = "hierarchy"
 path = "examples/ecs/hierarchy.rs"
 
+[[example]]
+name = "dynamic_systems"
+path = "examples/ecs/dynamic_systems.rs"
+
 [[example]]
 name = "breakout"
 path = "examples/game/breakout.rs"

crates/bevy_ecs/hecs/src/archetype.rs

@@ -241,7 +241,8 @@ impl Archetype {
         size: usize,
         index: usize,
     ) -> Option<NonNull<u8>> {
-        debug_assert!(index < self.len);
+        // TODO(zicklag): I'm pretty sure that it is valid for the index to be zero
+        debug_assert!(index < self.len || index == 0);
         Some(NonNull::new_unchecked(
             (*self.data.get())
                 .as_ptr()
@@ -438,8 +439,11 @@ impl Archetype {
     }
 
     /// How, if at all, `Q` will access entities in this archetype
-    pub fn access<Q: Query>(&self) -> Option<Access> {
-        Q::Fetch::access(self)
+    pub fn access<S: Default, Q: Query>(&self, state: &S) -> Option<Access>
+    where
+        Q::Fetch: for<'a> Fetch<'a, State = S>,
+    {
+        Q::Fetch::access(self, state)
     }
 }
 

crates/bevy_ecs/hecs/src/lib.rs

@@ -81,8 +81,9 @@ pub use bundle::{Bundle, DynamicBundle, MissingComponent};
 pub use entities::{Entity, EntityReserver, Location, NoSuchEntity};
 pub use entity_builder::{BuiltEntity, EntityBuilder};
 pub use query::{
-    Access, Added, BatchedIter, Changed, Mut, Mutated, Or, Query, QueryBorrow, QueryIter,
-    ReadOnlyFetch, With, Without,
+    Access, Added, BatchedIter, Changed, DynamicComponentAccess, DynamicComponentInfo,
+    DynamicComponentQuery, Mut, Mutated, Or, Query, QueryBorrow, QueryIter, ReadOnlyFetch, With,
+    Without,
 };
 pub use query_one::QueryOne;
 pub use world::{